Engine with compression and momentum stroke

ABSTRACT

A linear reciprocating engine may include a cylinder having a first combustion chamber at one end and a second combustion chamber at an opposing end, first and second cylinder heads located at an end of the first and second combustion chambers, respectively, and a double-faced piston slidably mounted within the cylinder. The engine may further include a first piston rod portion extending from a first face of the double-faced piston through the first combustion chamber, and a second piston rod portion extending from a second face of the piston through the second combustion chamber. Passageways in the piston rod portions may be configured to communicate gases between the combustion chamber and a location outside the cylinder and configured to prevent gases from being exchanged between the cylinder and a location outside the cylinder via a path that crosses both face of the piston.

RELATED APPLICATIONS

This application is a continuation application of and claims the benefitof priority from U.S. patent application Ser. No. 15/210,596, filed onJul. 14, 2016, entitled, “Gas Exchange Through Engine Piston Rod,” whichitself claims priority based on U.S. Provisional Application No.62/192,575, entitled “Free Piston Engine,” filed on Jul. 15, 2015,International Application PCT/IL2015/050425 filed on Apr. 22, 2015, andU.S. Provisional Application No. 61/983,469 filed on Apr. 24, 2014, allof which are expressly incorporated herein by reference in theirentirety.

TECHNICAL FIELD

The present disclosure relates to the field of internal combustionengines, and more particularly to the field of internal combustionengines having a free piston.

BACKGROUND

Internal combustion engines are known. The most common types of pistonengines are two-stroke engines and four-stroke engines. These types ofengines include a relatively large number of parts, and require numerousauxiliary systems, e.g., lubricant systems, cooling systems, intake andexhaust valve control systems, and the like, for proper functioning.

SUMMARY

Some embodiments of the disclosure may include a linear reciprocatinginternal combustion engine. The engine may include a cylinder having afirst combustion chamber at a first end thereof and a second combustionchamber at an opposing second end thereof, a first cylinder head locatedat an end of the first combustion chamber, a second cylinder headlocated at an end of the second combustion chamber, and a double-facedpiston slidably mounted within the cylinder. The piston may beconfigured to travel in a first stroke from a first position in a regionof the first combustion chamber to a second position in a region of thesecond combustion chamber. The double-faced piston may include a firstface and a second face. The engine may further include a first pistonrod portion extending from the first face of the double-faced pistonthrough the first combustion chamber, and a second piston rod portionextending from a second face of the piston through the second combustionchamber. A first recess in the first piston rod portion may form a firstpassageway configured to communicate gases between the first combustionchamber and a first location outside the cylinder opposite the firstcylinder head. A second recess in the second piston rod portion may forma second passageway configured to communicate gases between the secondcombustion chamber and a second location outside the cylinder oppositethe second cylinder head. The first passageway and the second passagewaymay be configured to prevent gases from being exchanged between thecylinder and a location outside the cylinder via a path that crosses thefirst face and the second face.

In some aspects of the disclosure, the first passageway and the secondpassageway may render the first and second piston rod portions at leastpartially hollow.

At least one of the first and second passageways may include a groove inthe respective first and second piston rod portions.

The first and second passageways may be configured to introducecombustion gas into the first and second combustion chambers,respectively, from a location outside the cylinder.

In some embodiments, the first and second passageways may includeelongated channels extending internal to the first and second piston rodportions. Further, the first and second piston rod portions may beintegrally formed. The first and second piston rod portions may beindirectly connected to each other through the double-faced piston.

The engine may further include at least one port in the first piston rodportion in fluid communication with the first passageway and at leastone port in the second piston rod portion in fluid communication withthe second passageway. The at least one port in the first and secondpiston rod portions may include multiple elongated slots. The at leastone port in the first and second piston rod portions may also includemultiple holes in the piston rod portions.

In some embodiments, at least one of the first and second passageways inthe first and second piston rod portions may include a plurality ofgrooves formed in an outer peripheral surface of the respective pistonrod portion.

At least one of the first and second recesses in the first and secondpiston rod portions may include a piston rod section of reduceddiameter.

The first passageway in the first piston rod portion and the secondpassageway in the second piston rod portion may be configured to intakegases into the first and second combustion chambers, respectively.

The forgoing generally describes just a few exemplary aspects of thedisclosure. It is to be understood that both the foregoing generaldescription and the following detailed description are exemplary andexplanatory only, and are not restrictive of the invention, as claimed.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of a free piston engine according to thepresent disclosure;

FIG. 2 is a partial cross-sectional view of the engine of FIG. 1 withthe piston at top dead center on a left side of the cylinder;

FIG. 3 is a partial cross-sectional view of the engine of FIG. 1 withthe piston in a momentum portion of the stroke, in an early stage ofcompressing gasses on the right side of the engine;

FIG. 4 is a partial cross-sectional view of the engine of FIG. 1 ascompression continues on a right side of the cylinder beyond thecompression illustrated in FIG. 3;

FIG. 5 is a partial cross-sectional view of the engine of FIG. 1 in anadvanced stage of compression on the right side of the cylinder beyondthe compression illustrated in FIG. 4;

FIG. 6 is a partial cross-sectional view of the engine of FIG. 1 in aneven more advanced stage of compression on the right side of thecylinder beyond the compression illustrated in FIG. 5;

FIG. 7 is a partial cross-sectional view of the engine of FIG. 1 withthe piston at top dead center on a right side of the cylinder;

FIG. 8 is a partial cross-sectional view of the engine of FIG. 1 withthe piston in a momentum portion of the stroke, in an early stage ofcompressing gasses on the left side of the cylinder;

FIG. 9 is a partial cross-sectional view of the engine of FIG. 1 ascompression continues on a left side of the cylinder beyond thecompression illustrated in FIG. 8;

FIG. 10 is a partial cross-sectional view of the engine of FIG. 1 in anadvanced stage of compression on the left side of the cylinder beyondthe compression illustrated in FIG. 9;

FIG. 11 is a partial cross-sectional view of the engine of FIG. 1 in aneven more advanced stage of compression on the left side of the cylinderbeyond the compression illustrated in FIG. 10;

FIG. 12, similar to FIG. 2, illustrates top dead center piston positionon the left side of the cylinder;

FIG. 13 is a perspective view of a piston assembly that may be used withthe engine of FIGS. 1 and 2;

FIG. 14 is a perspective view of a piston center disk of the pistonassembly of FIG. 13;

FIG. 15 is a perspective view of a left-side piston disk of the pistonassembly of FIG. 13;

FIG. 16 is a perspective view of a right-side piston disk of the pistonassembly of FIG. 13;

FIG. 17 is a perspective view of a piston ring that may be used with thepiston assembly of FIG. 13;

FIG. 18 is a side view of the piston ring of FIG. 17;

FIG. 19 is a plan view of the piston ring of FIG. 17;

FIG. 20 is a perspective view of the piston assembly of FIG. 13 with thepiston ring of FIG. 17;

FIG. 21 is a side view of the piston assembly and piston ring of FIG. 20assembled on the piston rods of FIG. 2;

FIG. 22 is another perspective view of the piston assembly and pistonring of FIG. 20 assembled on the piston rods of FIG. 2 with differentinlet passageways; and

FIG. 23 is a perspective partial cross-sectional view of the engine ofFIG. 1.

DETAILED DESCRIPTION

The present disclosure relates to internal combustion engines. While thepresent disclosure provides examples of free piston engines, it shouldbe noted that aspects of the disclosure in their broadest sense, are notlimited to a free piston engine. Rather, it is contemplated that theforgoing principles may be applied to other internal combustion enginesas well.

An internal combustion engine in accordance with the present disclosuremay include an engine block. The term “engine block,” also usedsynonymously with the term “cylinder block,” may include an integratedstructure that includes at least one cylinder housing a piston. In thecase of a free piston engine block, the engine block may include asingle cylinder, or it may include multiple cylinders.

In accordance with the present disclosure, a cylinder may define atleast one combustion chamber in the engine block. In some internalcombustion engines in accordance with the present disclosure, acombustion chamber may be located on a single side of a cylinder withinan engine block. In other internal combustion engines in accordance withthe present disclosure, the internal combustion engine may include twocombustion chambers, one on each side of a cylinder within an engineblock.

Embodiments of the present disclosure may further include a piston inthe cylinder. In accordance with some embodiments of the invention usedin a free piston engine, the piston may include two heads on oppositesides thereof. In some embodiments of the invention, the piston may beconsidered to be “slideably mounted” in the cylinder. This refers to thefact that the piston slides through the cylinder from one side of thecylinder to the other. While the present disclosure describes pistonexamples, the invention in its broadest sense is not limited to aparticular piston configuration or construction.

FIGS. 1 and 2 illustrate an exemplary embodiment of a free piston engine10 according to the present disclosure. The free piston engine 10, whichis sometimes referred to herein simply as an engine, is one example ofan internal combustion engine including an engine block 8. A cylinder 12defining at least one combustion chamber may be included in the engineblock and may have a central, longitudinal axis A, and a double-facedpiston 50 reciprocally mounted in the cylinder 12. The double-facedpiston 50 may be configured to travel in a first stroke from a first endof the cylinder to an opposite second end of the cylinder, and in asecond stroke from the second end of the cylinder back to the first endof the cylinder. FIGS. 7-12 illustrate an exemplary movement of thepiston 50 from a first end of the cylinder to a second end of thecylinder. At least one piston rod portion may be connected to the pistonrod and may extend from a location within the at least one combustionchamber to an area external to the cylinder. As used herein, the termpiston rod portion includes any portion of a rod or shaft, extendingfrom a piston. In some embodiments, a piston rod portion may be aportion of a unified structure passing all the way through the piston.In other embodiments, a piston rod portion may be a portion of a pistonrod that extends from only one face of a piston.

By way of example, in FIG. 3, a piston rod portion 42 may be connectedone face of the piston 50 and extend from a location within the at leastone combustion chamber to an area 45 external to the cylinder.Similarly, a second piston rod portion 43 may extend from an oppositeface of double sided piston 50, to another area 47 external to thecylinder 12. Piston rod portions 42 and 43 may be integral with eachother, or may be completely separate structures, each extending from anopposite side of piston 50.

An area external (e.g. areas 45 and 47) to the cylinder may include aninlet manifold at each end of the cylinder configured for supplyingcombustion gases to each of the combustion chambers at the opposite endsof the cylinder from one or more sources of the gases external to thecylinder, or an exhaust manifold configured for receiving combustiongases from the combustion chambers and directing the combustion gasesaway from the cylinder for exhaust aftertreatment. In this way, forexample, a passageway of the piston rod portion is configured tointroduce combustion gas into a combustion chamber from a locationoutside the cylinder. In one embodiment, the areas 45 and 47 external tothe cylinder may simply refer to any region on an opposite side of acylinder head 14, 15 from the cylinder 12, regardless of whether theregion is in direct contact with a cylinder head. It is contemplatedthat ports could be provide to introduce gases from a manifold or othersource located alongside the cylinder, rather than at ends of thecylinder. Thus, in a general sense, locations outside the cylinder maybe at either the ends of the cylinder, alongside the cylinder, or acombination of both.

In accordance with embodiments of the invention, each piston rod portionmay include at least one recess forming a passageway configured tocommunicate gas flow between the at least one combustion chamber and thearea external to the cylinder. As used herein, the term “recess” can bedefined by any structure or void capable of communicating gas flow. Itmay include, for example, a channel or conduit completely or partiallycontained within at least part of the piston rod portion. Or, the recessmay include one or more exposed grooves or other cut-outs in at leastpart of the piston rod portion.

For example, in some exemplary embodiments of an engine according tothis disclosure, the one or more recesses forming passageways in thepiston rod portions may render the piston rod portions 42 and 43 atleast partially hollow. In some variations, a passageway may include agroove or grooves formed along an external periphery of the piston rodportion. Still further variations may include different outer diametersections of the piston rod portions. Such area(s) of reduced diametermay provide one or more gaps through which gas may flow. Alternatively,the one or more recesses forming the passageways may include a channelextending internal to a piston rod portion. In yet a furtheralternative, the recess may render the piston rod portion hollow in someareas and partially hollow (e.g., via external groove, slot, etc.) inother areas. At least one port may be formed in each piston rod, influid communication with the passageway of the piston rod portion, tothereby permit gas to enter and/or exit the passageway through the port.

By way of example with reference to FIG. 22, each piston rod portion 42and 43 may include a recess 53, 55, respectively (e.g., hollowed outinternal portion of piston rod portions 42 and 43), forming a passagewayor channel configured to communicate gas flow between the combustionchambers 49 and 51 (see FIGS. 5 and 10, respectively) and respectiveareas 45 and 47 external to the cylinder 12. The hollowed out regionmay, for example, be a bore through a core of a piston rod portion.

As illustrated in FIG. 5, a first combustion chamber may be defined inregion 49, between a face of piston 50 and a first head 14 of cylinder12. Likewise, as illustrated in FIG. 10, a second combustion chamber 51may be defined between an opposing face of piston 50 and an opposinghead 15 of cylinder 12. Of course, it is to be understood that eachcombustion chamber is a variable region that essentially includes aswept volume on each side of the piston, and which is compressed as thepiston moves from one end of the cylinder to the opposite end of thecylinder.

The passageways or recesses 53 and 55, as illustrated in FIG. 22, areexemplary only. For example, as illustrated, the recesses extend justpast ports 44, terminating before reaching piston 50. Numerous otherconfigurations are contemplated within this disclosure. For example,recesses 53 and 55 could extend further toward the piston, all the wayto the piston, or may cross one face of the piston. In a preferredembodiment, passageways 53 and 55 are not in flow-communication witheach other.

In one exemplary embodiment shown in the figures, one or more ports 44,which may be arranged in two groups, i.e., an inner group 46, that isclosest to the piston 50, and an outer group 48, that is distal to theinner group 46. Ports 44 may be configured to serve as inlets forconveying gas into the cylinder via recesses 53 and 55. In lieu of twogroups of inlet ports, only one group of inlet ports 44 may be employed,or more than two groups of inlet ports 44 spaced along the piston rodportions 42 may be employed. Moreover, the inlet ports do notnecessarily need to be arranged in groups, so long as there issufficient opening to convey gases from the channels within the pistonrods, defined by recesses 53 and 55.

In accordance with some embodiments of the invention, a first passagewayand the second passageway in the piston rod portions may be configuredto prevent gases from being exchanged between the cylinder and alocation outside the cylinder via a path that crosses the first face andthe second face of the piston. For example, the pair of piston rodportions 42 and 43 extending from opposite faces of the double-facedpiston 50 may be integrally formed, or may be indirectly connected toeach other through the double-faced piston. However, no interconnectingflow passageway may be provided between the piston rods. In such aconstruction, no communication of gas flow may occur between thecylinder and a location outside the cylinder that crosses both the firstand second faces of the double-faced piston 50. Thus, the recessesand/or passageways in each piston rod portion may be separate from eachother and may extend through different piston rod portions.

If the cylinder head on each side of the engine block includes (e.g., isconnected to or is integrally formed with) an intake manifold, thepassageway in the first piston rod portion may be configured tocommunicate gas flow between the first combustion chamber and the intakemanifold at the first end of the cylinder, and the passageway in thesecond piston rod portion may be configured to communicate gas flowbetween the second combustion chamber and the intake manifold at thesecond end of the cylinder. Thus, for example, with reference to FIG.10, gases from combustion gas inlet chamber 32 of intake manifold 26 mayenter the combustion chamber as ports 46 and 48 bridge the cylinder head14.

A cylinder in accordance with embodiments of the invention may be closedat both ends. For example, the cylinder 12 of engine 10 may be closed atboth ends thereof by a cylinder heads 14 and 15, which may be connectedto the cylinder 12 by a plurality of bolts 16. As used herein, the term“closed” does not require complete closure. For example, despite thatthe cylinder heads may have openings therein through which piston rodportions 42 and 43 pass, the cylinder heads are still considered“closed” within the meaning of this disclosure.

A peripheral portion of the cylinder 12 may be provided with coolingfins 24. Alternative configurations of the engine 10 may include otherexternal or internal features that assist with the cooling of thecylinder, such as water passageways formed internally within thecylinder walls or jacketing at least portions of the cylinder walls forwater cooling, and other configurations of cooling fins or otherconductive and/or convective heat transfer enhancement featurespositioned along the exterior of a cylinder peripheral wall tofacilitate fluid cooling of the cylinder.

Also in accordance with exemplary embodiments of the invention, aperipheral wall of the cylinder between the first and second ends mayinclude at least one exhaust port. By way of example only, the cylinder12 may include at least one exhaust port 18 in a peripheral side wall ofthe cylinder 12 between the first and second ends of the cylinder. Inthe exemplary embodiment illustrated in FIGS. 2-12, a plurality ofdistributed exhaust ports 18 may be spaced about the circumference ofthe cylinder at approximately a midpoint of the cylinder 12 between theopposite ends of the cylinder. The exhaust ports 18 may be of anysuitable size, shape, and distribution so as to accomplish the functionof exhausting gases from the cylinder. One of more of the exhaust portsmay, for example, be located in an axial central region of the cylinderperipheral wall, as illustrated in the figures. Although the exemplaryembodiment shown in the figures is configured symmetrically, with theexhaust ports 18 located midway between the opposite ends of thecylinder, alternative embodiments may position the exhaust ports at oneor more radial planes intersecting the cylinder peripheral wall atlocations other than the exact midway point between the cylinder heads14.

In accordance with some exemplary embodiments of the invention, at leastone port may be configured to communicate gas flow between the firstcombustion chamber and outside the cylinder when the piston is on thesecond combustion chamber side of the at least one port, and may beconfigured to communicate gas flow between the second combustion chamberand outside the cylinder when the piston is on the first combustionchamber side of the at least one port. By way of example only, this canoccur when, as illustrated in FIG. 5, piston 50 is located to the rightof ports 18, enabling conveyance of gas flow through port 18, from thecombustion chamber to the left of the piston 50. Ports 18 enable gasflow to a location “outside” the combustion chamber. That outsidelocation may be on the side of the cylinder as illustrated, or conduits(not shown) associated with the engine might deliver the gases to otherlocations.

The inlet manifold 26 may be connected to or formed integrally with eachof the cylinder heads 14,15 at opposite ends of the cylinder 12. Theinlet manifold 26 may include a piston rod opening 28 that is axiallyaligned with the longitudinal axis A, and one or more inlet openings 30,which may be positioned at a distal end of the inlet manifold, as shown,or at any location along the outer periphery of the inlet manifold. Theone or more inlet openings 30 in inlet manifold 26 may be configured todirect inlet gases into the inlet manifold transversely to thelongitudinal axis A. An inner space of the inlet manifold 26 may definean inlet chamber 32. Although the inlet manifold of the exemplaryembodiment shown in FIGS. 1-12 and 23 is illustrated as having acylindrically-shaped configuration, alternative embodiments may provideone or more inlet manifolds with other shaped profiles or crosssections, or may incorporate the inlet manifolds at least partiallywithin the cylinder heads 14, 15 as one or more internal passagewaysdefined within each of the cylinder heads at each end of the cylinder12.

Each of the cylinder heads 14, 15 may further include one or moreinjectors 34 that open into an annular or toroidal-shaped recess 36formed in or contiguous with a flame face of a fire deck of eachcylinder head at each end of the cylinder 12 in facing relationship withthe combustion chambers at each end of the cylinder 12. Toroidal-shapedrecess 36 may impart swirl flow to fuel gas injected by injectors 34 tofacilitate more complete combustion of the gases within the combustionchambers. The cylinder heads 14, 15 may also include one or morecavities for accommodating and mounting one or more spark plugs 38, andbushings 40 for aligning, supporting, guiding, and sealing (by means ofa dedicated seal) a piston rod portion 42, 43 that is supported by, andpasses through each of the cylinder heads 14, 15 at opposite ends of thecylinder 12. This is one example of how piston rod portions may extendfrom faces of a double-faced piston through a combustion chamber.Regardless of the particular details of any aperture through which thepiston rods may extend at ends of the cylinder, a piston rod thatextends to at least an end of the cylinder is said to extend through acombustion chamber within the meaning of this disclosure.

A double-faced piston consistent with embodiments of the invention, maybe configured to travel in a first stroke from a first end of thecylinder to an opposite second end of the cylinder, and in a secondstroke from the second end of the cylinder back to the first end. Thislength of travel is illustrated, by way of example, in FIGS. 2-7, whereFIG. 2 represents an end of a first stroke, FIG. 7 represents an end ofa second stroke, and FIGS. 3-6 represent exemplary intermediatepositions.

According to various exemplary embodiments of the present disclosure,the piston may be sized relative to the cylinder to enable an expansionstroke portion of each stroke wherein the piston travels under gasexpansion pressure, and a momentum stroke portion of each stroke for theremainder of the stroke following the expansion stroke portion. Theexpansion stroke portion of each of the first and second strokes of thepiston is the portion of travel when the piston directly moves under theexpansion pressure of combustion. For example, the expansion portion ofa stroke may be defined as the portion from a Top Dead Center (TDC)position of the piston at each end of the cylinder to the point at whichcombustion gases may be exchanged between the combustion chamber inwhich ignition of combustion gases has just occurred and an areaexternal to the cylinder.

At the TDC position of the piston during each stroke, a clearance volumeremains between each of the opposite faces of the double-faced pistonand a respective end of the cylinder as closed off by the cylinder heads14, 15. The combustion gases that have been introduced into thecombustion chamber before the piston reaches TDC are compressed into theremaining clearance volume on that side of the piston between the pistonface and the fire deck of the cylinder head. The compressed gases, whichusually include a fuel/air mixture, may be ignited by either a spark, orby self-ignition resulting at least in part from the compression of thecombustion gases. The expansion stroke portion of each stroke occursafter the ignition of the compressed combustion gases as chemical energyfrom the combustion in each combustion chamber is converted intomechanical power of the piston. Simultaneously with the expansion strokeportion of each stroke on one side of the piston, gas flow may occur forsubstantially the entire expansion stroke portion between the combustionchamber on the opposite side of the piston and the intake manifold atthe opposite end of the cylinder, as well as the exhaust manifold 20located at a central peripheral portion of the cylinder.

At the beginning of an expansion stroke portion of a stroke from theleft end of the cylinder to the right end, as shown in FIG. 2, gas flowmay occur between the combustion chamber on the right side of the pistonand the inlet manifold 26 on the right side of the cylinder, and betweenthe combustion chamber on the right side of the piston and the exhaustmanifold 20 through the exhaust ports 18. The communication of gasesbetween the combustion chamber on the right side of the piston and theexhaust manifold may continue until the right face of the piston hasmoved past the centrally located exhaust ports 18, acting as an exhaustvalve and shutting off communication between the right combustionchamber and the exhaust manifold. Additionally, before the piston 50 haseven closed off the exhaust ports 18, the inlet ports 44 closest to theright face of the piston may have moved outside of the right combustionchamber, thereby closing off communication of gases between the rightinlet manifold 26 and the right combustion chamber through the rightpiston rod portion 42.

According to some embodiments, a length of the double-faced piston, alength of the cylinder, a location of the exhaust outlet, and a locationof a channel access opening in each of the first and second piston rodportions may be arranged such that when the piston is in a combustionstage in the first combustion chamber, the piston blocks the exhaustoutlet from communicating with the first combustion chamber and thechannel access opening in the first piston rod portion is outside of thefirst combustion chamber, while simultaneously the exhaust outlet is influid communication with the second combustion chamber, and the accessopening of the second channel is within the second combustion chamber.This may be accomplished by various alternative structures. By way ofexample only with reference to the figures, the length of thedouble-faced piston 50, the length of the cylinder 12, the location ofthe exhaust outlets 18, and the location of the inlet ports 44 in eachof the first and second piston rod portions 42, 43 extending fromopposite faces of the piston 50 may be arranged such that when thepiston is in a combustion stage in a first combustion chamber on oneside of the piston, the piston blocks the exhaust outlet fromcommunicating with the first combustion chamber. The closest inlet port44 to the one side of the piston remains outside of the first combustionchamber, thereby preventing communication of gases between the intakemanifold on that one side of the piston and the first combustionchamber.

Simultaneously, the exhaust outlet is in fluid communication with thesecond combustion chamber on the opposite side of the piston, and inletports 44 in the second piston rod portion 43 are located within thesecond combustion chamber. Similarly, when the piston is in anothercombustion stage in the second combustion chamber on the opposite sideof the piston, the piston blocks the exhaust outlet from communicatingwith the second combustion chamber. The closest inlet port 44 to thesecond side of the piston remains outside of the second combustionchamber, thereby preventing communication of gases between the intakemanifold on the second side of the piston and the second combustionchamber. Simultaneously, the exhaust outlet is in fluid communicationwith the first combustion chamber on the first side of the piston, andinlet ports 44 in the first piston rod portion 42 are located within thefirst combustion chamber.

Following an expansion stroke portion, the piston may continue to movein a momentum stroke portion for a remainder of the stroke. The momentumstroke portion of each stroke encompasses the remaining portion of thestroke following the expansion stroke portion. In accordance withembodiments of the disclosure, substantially the entire momentum strokeportion of the second stroke on the second combustion chamber side ofthe piston may coincide with compression of gases in the firstcombustion chamber. That is, the momentum that follows an expansionportion of the stroke in one combustion chamber is used to compressgasses in the other combustion chamber. This may be made possible by anengine structure where an end of an expansion in one combustion chamberdoes not correspond with a TDC position in an opposing combustionchamber. Rather, the engine design enables further piston travelfollowing an expansion portion of the stroke. In some embodiments, thefurther piston travel during the momentum portion of the stroke may beat least a width of the piston. In other embodiments it may be multipletimes a width of the piston. In yet other embodiments, it may be atleast a half a width of the piston.

During the momentum stroke portion of each stroke, gases may beexchanged between the combustion chamber where ignition of combustiongases has just occurred and an area external to the cylinder. Theexchange of gases may occur through a passageway in the piston rodportion connected to the piston and extending from a location within theat least one combustion chamber to an area external to the cylinder, andthrough the exhaust ports formed in the peripheral wall of the cylinder.By way of one example with reference to FIGS. 2-7, the positions of thepiston 50 and the piston rod portions 42 are shown during a first strokefrom the far left position of the piston in FIG. 2 to the far rightposition of the piston in FIG. 7. FIGS. 7-12 show the positions of thepiston 50 and the piston rod portions 42 during a second stroke from thefar right position of the piston in FIG. 7 to the far left position ofthe piston in FIG. 12. The far left and far right positions of thepiston in the cylinder 12 may be referred to as Top Dead Center (TDC)for the stroke in which the combustion gases have been compressed andignition of the gases at the beginning of a combustion phase isoccurring. When the piston is in the far left position of FIG. 2 andignition is occurring for the combustion gases that have been compressedinto a clearance volume between the left face of the piston and thecylinder head 15 at the left end of the cylinder, the piston is at TDCfor the stroke from the left end to the right end of the cylinder asviewed in FIGS. 2-7. Similarly, when the piston is in the far rightposition of FIG. 7 and ignition is occurring for the combustion gasesthat have been compressed into a clearance volume between the right faceof the piston and the cylinder head 14 at the right end of the cylinder,the piston is at TDC for the stroke from the right end to the left endof the cylinder as viewed in FIGS. 7-12.

As the piston continues to move from TDC for a stroke from the left endof the cylinder to the right end of the cylinder, FIG. 3 illustrates thepiston at a position where the piston has just passed the centrallylocated exhaust ports 18. At this point, a first combustion chamber onthe left side of the piston is now in fluid communication with thecentrally located exhaust ports 18 and exhaust gases from the combustionmay start to exit the combustion chamber. Therefore, the expansionstroke portion of the stroke has ended, and the piston is continuing totravel toward the right end of the cylinder in the momentum strokeportion as a result of inertia remaining after the end of the expansionstroke.

As shown in FIGS. 3 and 4, the double-faced piston 50, the first pistonrod portion 43 on the left side of the piston and the centrally locatedexhaust ports 18 may be configured such that the double-faced pistonpasses the centrally located exhaust ports 18 as the piston moves fromthe left end of the cylinder toward the right end of the cylinder beforethe inlet ports 44 closest to the left face of the piston enter thefirst combustion chamber on the left side of the piston. As shown inFIG. 4, the piston 50 has moved completely to the right of the centrallylocated exhaust ports 18 by the time inlet ports 44 in the left pistonrod portion 42 are entering the combustion chamber on the left side ofthe piston to permit gas flow between the combustion chamber and theinlet ports 44. This relative sizing and spacing of the variouscomponents allows exhaust gases generated in the first combustionchamber to begin exiting from the centrally located exhaust ports 18before fresh pre-compressed air or other combustion gases are introducedinto the first combustion chamber through the piston rod portion 43 onthe left side of the piston. In various alternative embodiments, theprecise placement of the inlet ports through piston rod portions 42, 43relative to the opposite faces of the double-faced piston may be variedsuch that the closest inlet port to each face of the piston enters therespective combustion chamber on the same side of the piston shortlyafter the face of the piston has passed the near edge of the centrallylocated exhaust ports, thereby allowing exhaust gases to begin exitingthe respective combustion chamber a short time before introduction ofthe fresh pre-compressed air or other combustion gases (see e.g., FIGS.4 and 9).

Shortly after the piston has passed the centrally located exhaust ports18 during the momentum stroke portion of the stroke from the left end ofthe cylinder to the right end of the cylinder, as shown in FIG. 4, theedges of the inlet ports 44 in the piston rod portion 43 that areclosest to the left face of the piston start to enter the leftcombustion chamber. At this point a scavenging phase may occur on theleft side of the piston as a result of pre-compressed gases beingintroduced into the left combustion chamber through the piston rodportion 43 and inlet ports 44. The inlet ports 44 are configured suchthat when the piston is in the momentum stroke portion of the firststroke from the left end to the right end of the cylinder, gas flow maybe continuously communicated between the left combustion chamber and anarea external to the cylinder. In the exemplary embodiment shown in thefigures, fresh, pre-compressed air may be introduced into the leftcombustion chamber from the intake manifold 26 located opposite thecylinder head or integral with the cylinder head on the left end of thecylinder. Simultaneously, exhaust gases may be scavenged from the leftcombustion chamber by the incoming pre-compressed air or other gases andforced out of the centrally located exhaust ports 18.

Some aspects of the invention may involve the cylinder and thedouble-faced piston being sized such that the expansion stroke portionof the first stroke on a first side of the piston as the piston movesfrom the first end of the cylinder to the second end of the cylindercoincides with at least one of a scavenging phase and a gas boost phaseon a second side of the piston. A similar coincidence may occur inconnection with the second stroke. By way of non-limiting example withreference to the figures, as the piston continues to move toward theright end of the cylinder, as shown in FIGS. 5 and 6, gas flow may becontinuously communicated between the left combustion chamber and anarea external to the cylinder. The continuous flow of pre-compressed airor other gases introduced from the inlet manifold 26 into the combustionchamber may assist with cooling of the cylinder as well as scavenging ofexhaust gases from the combustion chamber, and boosting the gas pressurewithin the left combustion chamber. A similar coincidence is illustratedfor the second stroke in FIGS. 11 and 12. In some embodiments, thecoincidence of compression on one side with scavenging and gas boost onthe other side may precisely correspond. In other embodiments they maysubstantially overlap.

Some aspects of the invention may involve the cylinder and thedouble-faced piston being sized such that the momentum stroke portion ofthe first stroke on a first side of the piston as the piston moves fromthe first end of the cylinder to the second end of the cylindercoincides with a compression phase in the second combustion chamber on asecond side of the piston. By way of non-limiting example,simultaneously with the momentum stroke portion of the first stroke fromthe left end of the cylinder to the right end of the cylinder, after thepiston has moved past the centrally located exhaust ports 18 toward theright end of the cylinder, gases on the right side of the piston arecompressed during a compression phase on the right side of the piston.When the piston is all the way to the right, as shown in FIG. 7, thecombustion gases on the right side of the piston will have beencompressed into the remaining clearance volume of the right combustionchamber and ignition will occur to begin the second stroke.

As best seen by way of non-limiting example in FIGS. 2-12, the cylinder12 and the double-faced piston 50 may be sized such that a totaldistance the piston travels during the first stroke from the left end ofthe cylinder to the right end of the cylinder, or during the secondstroke from the right end of the cylinder to the left end of thecylinder may be substantially greater than a distance the piston 50travels during the expansion stroke portion of either stroke. In someexemplary embodiments the cylinder and the double-faced piston may besized such that the total distance the piston travels during each strokefrom one end of the cylinder to the opposite end of the cylinder mayexceed the distance the piston travels during the expansion strokeportion of the stroke by at least the length of the piston from one faceto the opposite face. In other exemplary embodiments the cylinder andthe double-faced piston may be sized such that a total distance thepiston travels in each stroke exceeds by at least the length of thepiston a distance traveled by the piston during compression of gases onone side of the piston. The length of the piston 50 from one face to theopposite face in the exemplary embodiment shown in the figures may beless than ½ of a distance from at least one of the cylinder heads 14 tothe centrally located exhaust ports 18. This configuration and relativesizing of the piston and cylinder allows for a significantly greaterlength of the total stroke for the piston in each direction during whichfresh pre-compressed air or other gases may be introduced into thecylinder for the purposes of scavenging exhaust gases and cooling thecylinder after each combustion occurs at opposite ends of the cylinder.

At the beginning of an expansion stroke portion of a stroke from theright end of the cylinder to the left end, as shown in FIG. 7, gas flowmay occur between the combustion chamber on the left side of the pistonand the inlet manifold 26 on the left side of the cylinder, and betweenthe combustion chamber on the left side of the piston and the exhaustmanifold 20 through the exhaust ports 18. The communication of gasesbetween the combustion chamber on the left side of the piston and theexhaust manifold may continue until the left face of the piston hasmoved past the centrally located exhaust ports 18, acting as an exhaustvalve and shutting off communication between the left combustion chamberand the exhaust manifold. Additionally, before the piston 50 has evenclosed off the exhaust ports 18, the inlet ports 44 closest to the leftface of the piston will have moved outside of the left combustionchamber, thereby closing off communication of gases between the leftinlet manifold 26 and the left combustion chamber through the leftpiston rod portion 43.

The length of the double-faced piston 50, the length of the cylinder 12,the location of the exhaust outlets 18, and the location of the inletports 44 in each of the first and second piston rod portions 42, 43extending from opposite faces of the piston 50 may be arranged such thatwhen the piston is in a combustion stage in the second combustionchamber on the right side of the piston, the piston blocks the exhaustoutlet from communicating with the second combustion chamber. Theclosest inlet port 44 to the right side of the piston remains outside ofthe second combustion chamber, thereby preventing communication of gasesbetween the intake manifold on the right side of the piston and thesecond combustion chamber. Simultaneously, the exhaust outlet is influid communication with the first combustion chamber on the left sideof the piston, and inlet ports 44 in the left piston rod portion 43 arelocated within the first combustion chamber.

The momentum stroke portion of each stroke encompasses the remainingportion of the stroke following the expansion stroke portion. During themomentum stroke portion of each stroke, gases may be exchanged betweenthe combustion chamber where ignition of combustion gases has justoccurred and an area external to the cylinder. The exchange of gases mayoccur through a passageway in the piston rod portion connected to thepiston and extending from a location within the at least one combustionchamber to an area external to the cylinder, and through the exhaustports formed in the peripheral wall of the cylinder. FIGS. 7-12 show thepositions of the piston 50 and the piston rod portions 42 during asecond stroke from the far right position of the piston in FIG. 7 to thefar left position of the piston in FIG. 12. As discussed above, the farleft and far right positions of the piston in the cylinder 12 may bereferred to as Top Dead Center (TDC) for the stroke in which thecombustion gases have been compressed and ignition of the gases at thebeginning of a combustion phase is occurring. When the piston is in thefar right position of FIG. 7 and ignition is occurring for thecombustion gases that have been compressed into a clearance volumebetween the right face of the piston and the cylinder head 14 at theright end of the cylinder, the piston is at TDC for the stroke from theright end to the left end of the cylinder, as viewed in FIGS. 7-12.

As the piston continues to move from TDC for a stroke from the right endof the cylinder to the left end of the cylinder, FIG. 8 illustrates thepiston at a position where the piston has just passed the centrallylocated exhaust ports 18. At this point, the second combustion chamberon the right side of the piston is now in fluid communication with thecentrally located exhaust ports 18 and exhaust gases from the combustionthat occurred on the right side of the piston during the expansionstroke portion of the second stroke may start to exit the combustionchamber. Therefore, the expansion stroke portion of the second strokehas ended, and the piston is continuing to travel toward the left end ofthe cylinder in the momentum stroke portion as a result of inertiaremaining after the end of the expansion stroke.

As shown in FIGS. 8 and 9, the double-faced piston 50, the second pistonrod portion 42 on the right side of the piston and the centrally locatedexhaust ports 18 may be configured such that the double-faced pistonpasses the centrally located exhaust ports 18 as the piston moves fromthe right end of the cylinder toward the left end of the cylinder beforethe inlet ports 44 closest to the right face of the piston enter thesecond combustion chamber on the right side of the piston. As shown inFIG. 9, the piston 50 has moved completely to the left of the centrallylocated exhaust ports 18 by the time inlet ports 44 in the right pistonrod portion 42 are entering the second combustion chamber on the rightside of the piston to permit gas flow between the second combustionchamber and the inlet ports 44. This relative sizing and spacing of thevarious components allows exhaust gases generated in the secondcombustion chamber to begin exiting from the centrally located exhaustports 18 before fresh pre-compressed air or other combustion gases areintroduced into the second combustion chamber through the piston rodportion 42 on the right side of the piston. In various alternativeembodiments, the precise placement of the inlet ports through piston rodportions 42, 43 relative to the opposite faces of the double-facedpiston may be varied such that the closest inlet port to each face ofthe piston enters the respective combustion chamber on the same side ofthe piston shortly after the face of the piston has passed the near edgeof the centrally located exhaust ports, thereby allowing exhaust gasesto begin exiting the respective combustion chamber a short time beforeintroduction of the fresh pre-compressed air or other combustion gases.

Shortly after the piston has passed the centrally located exhaust ports18 during the momentum stroke portion of the stroke from the right endof the cylinder to the left end of the cylinder, as shown in FIG. 9, theedges of the inlet ports 44 in the piston rod portion 42 that areclosest to the right face of the piston start to enter the secondcombustion chamber. At this point a scavenging phase may occur on theright side of the piston as a result of pre-compressed gases beingintroduced into the second combustion chamber through the piston rodportion 42 and inlet ports 44. The inlet ports 44 are configured suchthat when the piston is in the momentum stroke portion of the secondstroke from the right end to the left end of the cylinder, gas flow maybe continuously communicated between the second combustion chamber andan area external to the cylinder. In the exemplary embodiment shown inthe figures, fresh, pre-compressed air may be introduced into the secondcombustion chamber from the intake manifold 26 located opposite thecylinder head or integral with the cylinder head on the right end of thecylinder. Simultaneously, exhaust gases may be scavenged from the secondcombustion chamber on the right side of the piston 50 by the incomingpre-compressed air or other gases and forced out of the centrallylocated exhaust ports 18.

As the piston continues to move toward the left end of the cylinder, asshown in FIGS. 10 and 11, gas flow may be continuously communicatedbetween the second combustion chamber and an area external to thecylinder. The continuous flow of pre-compressed air or other gasesintroduced from the inlet manifold 26 into the second combustion chambermay assist with cooling of the cylinder as well as scavenging of exhaustgases from the second combustion chamber, and boosting the gas pressurewithin the second combustion chamber. Simultaneously with the momentumstroke portion of the second stroke from the right end of the cylinderto the left end of the cylinder, after the piston has moved past thecentrally located exhaust ports 18 toward the left end of the cylinder,gases on the left side of the piston are compressed during a compressionphase on the left side of the piston. When the piston is all the way tothe left, as shown in FIG. 2, the combustion gases on the left side ofthe piston will have been compressed into the remaining clearance volumeof the left combustion chamber and ignition will occur to begin anotherstroke from the left end of the cylinder to the right end of thecylinder.

In accordance with some embodiments of the invention, regardless ofother particular structures in the engine, a cylinder and a double-facedpiston may be sized such that a total distance the piston travels duringa first stroke is substantially greater than a distance the pistontravels during an expansion stroke portion of the first stroke. By wayof example with reference to FIGS. 7-12, the total distance of pistontravel may be measured from TDC on the right side of the engine 10, asillustrated in FIG. 7, to TDC on the left side of engine 10, asillustrated in FIG. 12. This total distance traveled is substantiallygreater than the expansion portion of the stroke which occurs when, inthe progression of FIGS. 7-12, the piston 50 passes at least one of theexhaust ports 18. It is contemplated that in other embodiments of theinvention, the end of the expansion stroke might be marked by otheroccurrences, such as the opening of a mechanical valve, or the cessationof expansion in some other manner. Regardless of how the expansionstroke portion ends, such embodiments are contemplated to be within thescope of this disclosure so long as the total distance of travel issubstantially greater than the expansion portion alone. By way ofnon-limiting examples, the total distance may be consideredsubstantially greater if the difference between the expansion portion ofthe stroke and a non-expansion portion of the stroke is either multipletimes the width of the piston, the width of the piston, greater thanthree quarters the width of the piston, greater than half the width ofthe piston, or greater than a quarter width of the piston. Thus, forexample, the double-faced piston may have an axial length from one faceof the piston to an opposite face of the piston that is less than orequal to ½ of a distance from at least one of the first cylinder headand the second cylinder head to the exhaust port.

In some exemplary embodiments the cylinder and the double-faced pistonmay be sized such that the total distance the piston travels during eachstroke from one end of the cylinder to the opposite end of the cylindermay exceed the distance the piston travels during the expansion strokeportion of the stroke by at least the length of the piston from one faceto the opposite face. In other exemplary embodiments the cylinder andthe double-faced piston may be sized such that a total distance thepiston travels in each stroke exceeds by at least the length of thepiston a distance traveled by the piston during compression of gases onone side of the piston. The length of the piston 50 from one face to theopposite face in the exemplary embodiment shown in the figures may beless than ½ of a distance from at least one of the cylinder heads 14 tothe centrally located exhaust ports 18. This configuration and relativesizing of the piston and cylinder may allow for a significantly greaterlength of the total stroke for the piston in each direction during whichfresh pre-compressed air or other gases may be introduced into thecylinder for the purposes of scavenging exhaust gases and cooling thecylinder after each combustion occurs at opposite ends of the cylinder.

In accordance with some embodiments of the invention, an internalcombustion engine may include a piston being formed of an assembly ofseparate pieces, including a pair of piston end disks, each having afirst outer diameter, and wherein the center disk is configured to causea thermal gap between the pair of piston end disks. By way of example,and as shown in FIGS. 13 to 22, various embodiments of an engineaccording to this disclosure may include a double-faced piston 50. Thepiston 50 may include a cylindrical first piston portion 56 having afirst diameter, a cylindrical second piston portion 54 of the firstdiameter, and a cylindrical third piston portion 52 of a second diameterless than the first diameter. The cylindrical third piston portion 52may be located between the first piston portion 56 and the second pistonportion 54, and the first piston portion 56 may be configured such thatprior to assembly, the first piston portion 56 is separate from thesecond piston portion 52.

In accordance with some embodiments, the hardness of the center diskdiffers from the hardness of the end disks. In addition, oralternatively, the piston center disk may be integrally formed with oneof the pair of piston end disks.

Embodiments may also include a continuous, gapless piston ringcircumscribing a piston portion, where the piston ring is configuredsuch that when heated the piston ring deforms in an axial direction ofthe piston. Variously shaped piston rings may be employed consistentwith embodiments of the invention. Such shapes may include a wavepattern or other meandering constructions that are either symmetrical ornon-symmetrical. As illustrated by way of example only in FIG. 20, acontinuous, gapless piston ring 64 may circumscribe the third pistonportion 52, where the piston ring 64 is configured such that whenheated, the piston ring deforms in an axial direction of the piston 50.The third piston portion 52 may define a slot between the first pistonportion 56 and the second piston portion 54. The slot defined betweenthe first piston portion 56 and the second piston portion 54 may alsoform a thermal gap that is not completely filled by the piston ring, andthat therefore facilitates heat transfer away from the piston ring,thereby increasing its longevity. In some embodiments prior to assembly,the third piston portion 52 may be integral with the first pistonportion 56, and the second piston portion 54 may be non-integral withthe third piston portion 52.

As shown in FIG. 13, a groove in the outer peripheral wall of the piston50 may be defined by the assembly of the first, second, and third pistonportions, as described above, or may be machined or otherwisemanufactured, e.g., using 3D additive manufacturing processes. Thegroove may include a first edge and a second edge spaced from the firstedge. A piston ring 64 (FIGS. 17-20) may be installed in the groove, andthe piston ring may have a shape that meanders within the groove, suchthat the shape of the piston ring differs from a shape of the groove andsuch that the piston ring does not substantially fill the groove. Thepiston ring 64 may be constructed of a material that when subjected toheat causes a shape of the meanderings to change, thereby enabling thepiston ring to expand in an axial direction of the piston, between theedges of the groove. As best seen in FIGS. 17, 19, and 20, themeanderings of the piston ring 64 may be in the shape of a wave. Peaksof the wave alternatively extend toward opposing edges of the groove.The piston ring 64 may be constructed such that when subjected to heat,the piston ring tends to expand in an axial direction of the pistonrather than radially.

As shown in FIGS. 17-20, the piston ring 64 may have an undulating axialcross section and a circular radial cross section. The piston ring 64may include a plurality of staggered, flat abutment surface portions 68on axially opposite faces. The flat abutment surface portions 68 may beconfigured to seat alternately on opposite edges of the groove. A gapbetween the first and second edges of the groove of the piston 50 mayallow for axially-directed expansion and contraction of the piston ring64 while maintaining a circular radial cross section of the piston ringhaving a substantially constant outer diameter 70 that remains in fullcontact with an inner peripheral wall of the cylinder 12 at all times.

In a plan view of the piston ring 64, as can be clearly seen in FIG. 19,the piston ring 64 is round, in order to fit tightly against a cylinderwall 66. In one exemplary embodiment, each side of the piston ring 64may be provided with six evenly peripherally distributed flat abutmentsurface portions 68 for abutting the piston ring 64 against the adjacentpiston portion, i.e., the first piston portion 56 and the second pistonportion 54. The abutment surface portions 68 of one side of the pistonring 64 may be angularly shifted with respect to the abutment surfaceportions 68 of the other side of the piston ring 64, such that eachabutment surface portion 68 of one side of the piston ring 64 is equallydistanced from the two adjacent abutment surface portions 68 of theother side of the piston ring 64.

As can be seen in FIG. 18, which is a side view of the piston ring 64, acurved ring wall 69 may be formed between two adjacent abutment surfaceportions 68 of both sides of the piston ring 64.

Depending on construction and materials employed, in some embodimentsthe above described structure of the piston ring 64 may have severaladvantages. The piston ring 64 is peripherally continuous, in contrastto traditional piston rings, thus substantially eliminating compressionlosses during the operation of the engine due to leakage of compressedgas from one side of the piston ring to an opposite side thereof. As aresult of the reduction in compression losses, a single piston ring 64may be used, rather than two or three piston rings, as known in the art.(although multiple rings consistent with this disclosure may be employedon a single piston consistent with this disclosure.) The reduction inthe number of piston rings may result in a significant reduction infriction losses caused by the sliding contact between each piston ringand the cylinder wall 66. The reduction in friction losses in turn mayresult in improvements in the efficiency of the engine 10. The abutmentsurface portions 68 on both sides of the piston ring 64 may also ensurethat the piston ring 64 will remain directed in an orientationsubstantially perpendicular to the longitudinal axis A, which in turnmay result in the ring peripheral surface 70 remaining parallel to thecylinder wall 66 and in a continuous contact therewith. As the pistonring 64 according to various exemplary embodiments of this disclosure isheated during operation and tends to expand, the ring peripheral surface70 will remain in full contact with the cylinder wall 66, and may exertsubstantially consistent pressure thereon. Expansion and contraction ofthe piston ring 64 may result in an increased curvature andaxially-directed expansion of the curved ring walls 69, therebyabsorbing the expansion without disturbing the constant radial profileof the piston ring 64.

The engine 10 according to the various exemplary embodiments of thisdisclosure may facilitate a nearly continuous scavenging of hot exhaustgases from the engine while continuously supplying fresh air forcombustion. The nearly continuously introduced fresh pre-compressed airmay decrease the temperature within the cylinder and increase the engineefficiency and engine service life.

To expedite the foregoing portion of the disclosure, variouscombinations of elements are described together. It is to be understood,that aspects of the invention in there broadest sense are not limited tothe particular combinations previously disclosed. Rather, embodiments ofthe invention, consistent with this disclosure, and as illustrated byway of example only in the Figures, may include one or more of thefollowing, either alone or in combination with any one or more other ofthe following, or in combination with the previously disclosed features:

-   -   an internal combustion engine.    -   a cylinder defining at least one combustion chamber in the        engine block.    -   a piston in the cylinder, the piston being configured to travel        in a first stroke from one end of the cylinder to an opposite        end of the cylinder, and being sized relative to the cylinder to        enable an expansion stroke portion of the first stroke wherein        the piston travels under gas expansion pressure, and a momentum        stroke portion of the first stroke for the remainder of the        first stroke following the expansion stroke portion.    -   at least one piston rod portion connected to the piston and        extending from a location within the at least one combustion        chamber to an area external to the cylinder.    -   at least one recess in the piston rod portion, the at least one        recess forming a passageway configured to communicate gas flow        between the at least one combustion chamber and the area        external to the cylinder.    -   wherein the at least one recess is configured such that when the        piston is in the momentum stroke portion of the first stroke        following the expansion stroke portion of the first stroke, the        at least one recess is configured to continuously communicate        gas flow between the at least one combustion chamber and the        area external to the cylinder.    -   wherein the at least one recess forming the passageway renders        the at least one piston rod portion at least partially hollow.    -   wherein the passageway includes a groove in the at least one        piston rod portion.    -   wherein the passageway is configured to introduce combustion gas        into the at least one combustion chamber from a location outside        the cylinder.    -   wherein the piston is double-faced and wherein the at least one        piston rod portion includes a pair of piston rod portions, each        piston rod portion extending from an opposing face of the        double-faced piston.    -   wherein the at least one recess includes a channel extending        internal to the at least one piston rod portion.    -   wherein the pair of piston rod portions are integrally formed.    -   wherein the pair of piston rod portions are indirectly connected        to each other through the double-faced piston.    -   wherein the at least one recess includes at least two recesses,        each extending through a different piston rod portion.    -   further including at least one port in the at least one piston        rod portion and in fluid communication with the passageway.    -   wherein the at least one port includes multiple elongated slots.    -   wherein the at least one port includes multiple holes in the        piston rod.    -   wherein the passageway includes a plurality of grooves formed in        an outer peripheral surface of the at least one piston rod        portion.    -   wherein the at least one recess in the piston rod portion        includes a rod section of reduced diameter.    -   wherein the at least one combustion chamber includes a first        combustion chamber defined between a first end of the piston and        a first end of the cylinder, and a second combustion chamber        defined between a second end of the piston and a second end of        the cylinder.    -   wherein the cylinder is closed at each opposite end by a        cylinder head.    -   wherein the at least one piston rod portion includes a first        piston rod portion extending from the first end of the piston        through the cylinder head at the first end of the cylinder, and        a second piston rod portion extending from the second end of the        piston through the cylinder head at the second end of the        cylinder.    -   wherein the cylinder head at each end of the cylinder includes        an intake manifold, wherein the passageway in the first piston        rod portion is configured to communicate gas flow between the        first combustion chamber and the intake manifold at the first        end of the cylinder, and the passageway in the second piston rod        portion is configured to communicate gas flow between the second        combustion chamber and the intake manifold at the second end of        the cylinder.    -   wherein a peripheral wall of the cylinder between the first and        second ends of the cylinder includes at least one exhaust port.    -   wherein the at least one exhaust port includes a plurality of        exhaust ports spaced around the circumference of the cylinder,        and wherein the plurality of exhaust ports are in fluid        communication with an exhaust manifold.    -   wherein substantially the entire expansion stroke portion of the        first stroke on the first combustion chamber side of the piston        coincides with gas flow between the second combustion chamber        and the intake manifold at the second end of the cylinder.    -   wherein substantially the entire momentum stroke portion of the        first stroke on the first combustion chamber side of the piston        coincides with compression of gases in the second combustion        chamber.    -   wherein the piston is further configured to travel in a second        stroke from the second end of the cylinder to the first end of        the cylinder, and being sized relative to the cylinder to enable        an expansion stroke portion of the second stroke wherein the        piston travels under gas expansion pressure, and a momentum        stroke portion of the second stroke for the remainder of the        second stroke following the expansion stroke portion.    -   wherein substantially the entire expansion stroke portion of the        second stroke on the second combustion chamber side of the        piston coincides with gas flow between the first combustion        chamber and the intake manifold at the first end of the        cylinder.    -   wherein substantially the entire momentum stroke portion of the        second stroke on the second combustion chamber side of the        piston coincides with compression of gases in the first        combustion chamber.    -   a double-faced piston slidably mounted within the cylinder and        configured to move in a first stroke from the first end of the        cylinder to the second end of the cylinder, wherein the        double-faced piston and the cylinder are configured such that        the first stroke includes an expansion stroke portion during        which chemical energy from combustion in the first combustion        chamber is converted into mechanical power of the piston, and a        momentum stroke portion during which the piston continues to        move to the second end of the cylinder and gases are exchanged        between the first combustion chamber and a location outside the        cylinder.    -   wherein the cylinder and the double-faced piston are sized such        that a total distance the piston travels during the first stroke        is substantially greater than a distance the piston travels        during the expansion stroke portion of the first stroke.    -   wherein the cylinder and the double-faced piston are sized such        that the total distance the piston travels during the first        stroke exceeds the distance the piston travels during the        expansion stroke portion of the first stroke by at least the        length of the piston from one face to the opposite face.    -   wherein the cylinder and the double-faced piston are sized such        that the expansion stroke portion of the first stroke on a first        side of the piston as the piston moves from the first end of the        cylinder to the second end of the cylinder coincides with at        least one of a scavenging phase and a gas boost phase on a        second side of the piston.    -   wherein the cylinder and the double-faced piston are sized such        that the momentum stroke portion of the first stroke on a first        side of the piston as the piston moves from the first end of the        cylinder to the second end of the cylinder coincides with a        compression phase in the second combustion chamber on a second        side of the piston.    -   wherein the double-faced piston is configured to move in a        second stroke from the second end of the cylinder to the first        end of the cylinder, and wherein the cylinder and the        double-faced piston are sized such that the second stroke        includes an expansion stroke portion during which chemical        energy from combustion in the second combustion chamber is        converted into mechanical power of the piston, and a momentum        stroke portion during which the piston continues to move to the        first end of the cylinder and gases are exchanged between the        second combustion chamber and a location outside the cylinder.    -   wherein the cylinder and the piston are sized such that a total        distance the piston travels during the second stroke is        substantially greater than a distance the piston travels during        the expansion portion of the second stroke.    -   wherein the total distance the piston travels during the second        stroke exceeds the distance the piston travels during the        expansion stroke portion of the second stroke by at least the        length of the piston from one face to the opposite face.    -   wherein the expansion stroke portion of the second stroke on a        second side of the piston as the piston moves from the second        end of the cylinder to the first end of the cylinder coincides        with at least one of a scavenging phase and a gas boost phase on        a first side of the piston.    -   wherein the momentum portion of the second stroke on a second        side of the piston as the piston moves from the second end of        the cylinder to the first end of the cylinder coincides with a        compression phase in the first combustion chamber on a first        side of the piston.    -   a first piston rod portion connected to a first face of the        double-faced piston and extending from a location within the        first combustion chamber to a first location outside the        cylinder.    -   a second piston rod portion connected to a second face of the        double-faced piston and extending from a location within the        second combustion chamber to a second location outside the        cylinder.    -   at least one recess in the first piston rod portion, the at        least one recess forming a passageway configured to communicate        gas flow between the first combustion chamber and the first        location outside the cylinder.    -   at least one recess in the second piston rod portion, the at        least one recess forming a passageway configured to communicate        gas flow between the second combustion chamber and the second        location outside the cylinder.    -   at least one port in a peripheral side wall of the cylinder, the        at least one port being configured to communicate gas flow        between the first combustion chamber and outside the cylinder        when the piston is on the second combustion chamber side of the        at least one port, and being configured to communicate gas flow        between the second combustion chamber and outside the cylinder        when the piston is on the first combustion chamber side of the        at least one port.    -   wherein the passageways in the first and second piston rod        portions are configured to intake gases into the first and        second combustion chambers, respectively, and the at least one        port in a peripheral side wall of the cylinder is configured to        exhaust gases from the first and second combustion chambers,        respectively.    -   wherein each of the first stroke and the second stroke includes        an expansion stroke portion during which chemical energy from        combustion in one of the first combustion chamber and the second        combustion chamber is converted into mechanical power of the        piston, and a momentum stroke portion during which the piston        continues to move toward a respective end of the cylinder and        gases are exchanged between one of the first combustion chamber        and the second combustion chamber and a location outside the        cylinder.    -   wherein the cylinder and the piston are sized such that a total        distance the piston travels in each of the first and second        strokes exceeds by at least a length of the piston a distance        traveled by the piston during compression of gases on one side        of the piston.    -   wherein the cylinder and the piston are sized such that an        expansion stroke portion of the first stroke on a first side of        the piston as the piston moves from the first end of the        cylinder to the second end of the cylinder coincides with at        least one of a scavenging phase and a gas boost phase on a        second side of the piston.    -   wherein the cylinder and the piston are sized such that a        momentum stroke portion of the first stroke on a first side of        the piston as the piston moves from the first end of the        cylinder to the second end of the cylinder coincides with        compression of gases in the second combustion chamber on a        second side of the piston.    -   at least one port in a peripheral side wall of the cylinder, the        at least one port being configured to communicate gas flow        between the first combustion chamber and outside the cylinder        when the piston is on the second combustion chamber side of the        at least one port, and being configured to communicate gas flow        between the second combustion chamber and outside the cylinder        when the piston is on the first combustion chamber side of the        at least one port.    -   wherein the passageways in the first and second piston rod        portions are configured to intake gases into the first and        second combustion chambers, respectively, and the at least one        port in a peripheral side wall of the cylinder is configured to        exhaust gases from the first and second combustion chambers,        respectively.    -   wherein the first passageway and the second passageway are        configured to prevent gases from being exchanged between the        cylinder and a location outside the cylinder via a path that        crosses the first face and the second face.    -   wherein the first passageway and the second passageway render        the first and second piston rod portions at least partially        hollow.    -   wherein at least one of the first and second passageways        includes a groove in the respective first and second piston rod        portions.    -   wherein the first and second passageways are configured to        introduce combustion gas into the first and second combustion        chambers, respectively, from a location outside the cylinder.    -   wherein the first and second passageways include elongated        channels extending internal to the first and second piston rod        portions.    -   wherein the first and second piston rod portions are integrally        formed.    -   wherein the first and second piston rod portions are indirectly        connected to each other through the double-faced piston.    -   further including at least one port in the first piston rod        portion in fluid communication with the first passageway and at        least one port in the second piston rod portion in fluid        communication with the second passageway.    -   wherein the at least one port in the first and second piston rod        portions includes multiple elongated slots.    -   wherein the at least one port in the first and second piston rod        portions includes multiple holes in the piston rod portions.    -   wherein at least one of the first and second passageways in the        first and second piston rod portions includes a plurality of        grooves formed in an outer peripheral surface of the respective        piston rod portion.    -   wherein at least one of the first and second recesses in the        first and second piston rod portions includes a piston rod        section of reduced diameter.    -   wherein the first passageway in the first piston rod portion and        the second passageway in the second piston rod portion are        configured to intake gases into the first and second combustion        chambers, respectively.    -   a first piston rod portion extending from a first face of the        double-faced piston through the first combustion chamber and        through the first cylinder head.    -   a first recess in the first piston rod portion defining a first        passageway for communicating gas between the first combustion        chamber and a first location external to the cylinder.    -   a second piston rod portion extending from a second face of the        piston through the second combustion chamber and through the        second cylinder head.    -   a second recess in the second piston rod portion defining a        second passageway for communicating gas between the second        combustion chamber and a second location external to the        cylinder.    -   at least one port in a peripheral wall of the cylinder, for        alternatively communicating gases between at least one region        external to the cylinder and at least one of the first        combustion chamber and the second combustion chamber.    -   wherein the double-faced piston, the first piston rod portion,        and the at least one port are configured such that the        double-faced piston passes the at least one port as the piston        moves from the first position toward the second position before        an opening of the first recess enters the first combustion        chamber to thereby permit gas flow between the first combustion        chamber and the first recess in the first piston rod portion.    -   wherein the double-faced piston, the second piston rod portion,        and the at least one port are configured such that the        double-faced piston passes the at least one port as the piston        moves from the second position toward the first position before        an opening of the second recess enters the second combustion        chamber to thereby permit gas flow between the second combustion        chamber and the second recess in the second piston rod portion.    -   wherein the first recess in the first piston rod portion and the        second recess in the second piston rod portion are configured as        inlets for the intake of gases, and the at least one port in the        peripheral wall is configured as an outlet for the exhaust of        gases.    -   further including at least one additional recess in the first        piston rod portion and at least one additional recess in the        second piston rod portion.    -   wherein the double-faced piston, the first piston rod portion,        and the at least one port in the peripheral cylinder wall are        configured such that when the double-faced piston is located        between the first cylinder head and the at least one port in the        peripheral wall, an opening of the first recess is outside the        cylinder and the double-faced piston blocks gas flow between the        first combustion chamber and the at least one port, and wherein        the double-faced piston, the second piston rod portion, and the        at least one port in the peripheral cylinder wall are configured        such that when the double-faced piston is located between the        second cylinder head and the at least one port in the peripheral        wall, an opening of the second recess is outside the cylinder        and the double-faced piston blocks gas flow between the second        combustion chamber and the at least one port in the peripheral        wall.    -   wherein the recesses in the first piston rod portion and the        second piston rod portion include a bore through a respective        core of each of the first piston rod portion and the second        piston rod portion.    -   wherein the openings of the recesses in the first piston rod        portion and the second piston rod portions include a curvilinear        port in a respective outer wall of each respective piston rod        portion.    -   wherein the openings of the recesses in the first piston rod        portion and the second piston rod portions include an elongated        slot in a respective outer wall of each respective piston rod        portion.    -   wherein the recesses in the first and second piston rod portions        are defined by regions of reduced diameter.    -   wherein the at least one port includes an exhaust port located        in an axially central region of the cylinder peripheral wall.    -   wherein during compression and combustion of gases in one of the        first and second combustion chambers, the piston acts as an        exhaust valve preventing the flow of exhaust gases out of the        one of the combustion chambers while enabling the flow of        exhaust gases out of the other of the combustion chambers.    -   an exhaust port located in a peripheral wall of the cylinder at        a generally central region of the cylinder between the first        cylinder head and the second cylinder head.    -   at least one combustion gas inlet in a location other than the        peripheral cylinder wall, wherein the combustion gas inlet and        the exhaust port are configured to cooperate such that        combustion gases introduced through the inlet are evacuated from        the cylinder through the exhaust port in the peripheral wall.    -   wherein the double-faced piston has an axial length from one        face of the piston to an opposite face of the piston that is        less than or equal to ½ of a distance from at least one of the        first cylinder head and the second cylinder head to the exhaust        port.    -   further including a first piston rod portion extending from a        first face of the double-faced piston through the first        combustion chamber and through the first cylinder head, and        wherein the at least one combustion gas inlet is located in the        first piston rod portion.    -   further including a second piston rod portion extending from a        second face of the double-faced piston through the second        combustion chamber and through the second cylinder head, and        wherein the at least one combustion gas inlet is located in the        second piston rod portion.    -   wherein the at least one combustion gas inlet includes a first        passageway in fluid communication with a first intake manifold        located adjacent the first cylinder head and a second passageway        in fluid communication with a second intake manifold located        adjacent the second cylinder head.    -   a first elongated channel in the first piston rod portion        configured to serve as an intake inlet for gas from a location        external to the cylinder, through the first end of the first        combustion chamber to a location within the first combustion        chamber.    -   a second elongated channel in the second piston rod portion        configured to serve as an intake inlet for gas from a location        external to the cylinder, through the second end of the second        combustion chamber to a location within the second combustion        chamber.    -   wherein a length of the double-faced piston, a length of the        cylinder, a location of the exhaust outlet, and a location of a        channel access opening in each of the first and second piston        rod portions are arranged such that when the piston is in a        combustion stage in the first combustion chamber, the piston        blocks the exhaust outlet from communicating with the first        combustion chamber and the channel access opening in the first        piston rod portion is outside of the first combustion chamber,        while simultaneously the exhaust outlet is in fluid        communication with the second combustion chamber, and the access        opening of the second channel is within the second combustion        chamber.    -   wherein a spacing between a channel access opening in the first        piston rod portion and the first face of the piston and the        location of the exhaust outlet are configured such that        scavenging of combustion gases from the first combustion chamber        occurs through the exhaust outlet when a channel access opening        in the first piston rod portion is located within the first        combustion chamber and the piston is in a position on the second        combustion chamber side of the exhaust outlet.    -   wherein a spacing between a channel access opening in the first        piston rod portion and the first face of the piston and the        location of the exhaust outlet are configured such that gas        boost in the first combustion chamber follows scavenging of        combustion gases from the first combustion chamber as        pre-charged air continues to be introduced through the channel        access opening in the first piston rod portion into the first        combustion chamber.    -   wherein a spacing between a channel access opening in the second        piston rod portion and the second face of the piston and the        location of the exhaust outlet are configured such that        compression of gases within the second combustion chamber occurs        when the piston is in a position past the exhaust outlet toward        the second end of the second combustion chamber and the channel        access opening in the second piston rod portion is outside of        the second combustion chamber.    -   wherein a spacing between a channel access opening in the second        piston rod portion and the second face of the piston and the        location of the exhaust outlet are configured such that        scavenging of combustion gases from the second combustion        chamber occurs through the exhaust outlet when the channel        access opening in the second piston rod portion is in the second        combustion chamber and the piston is in a position past the        exhaust outlet toward the first end of the first combustion        chamber.    -   wherein a spacing between a channel access opening in the second        piston rod portion and the second face of the piston and the        location of the exhaust outlet is configured such that gas boost        in the second combustion chamber follows scavenging of        combustion gases from the second combustion chamber as        pre-charged air continues to be introduced through the channel        access opening in the second piston rod portion into the second        combustion chamber.    -   wherein a spacing between a channel access opening in the first        piston rod portion and the first face of the piston and the        location of the exhaust outlet is configured such that        compression of gases within the first combustion chamber occurs        when the piston is in a position past the exhaust toward the        first end of the first combustion chamber and the access opening        in the first piston rod portion is outside of the first        combustion chamber.    -   wherein a compression ratio of the engine is a function of at        least one of a closest spacing between a channel access opening        in the first piston rod portion and the first face of the        double-faced piston, and the closest spacing between a channel        access opening in the second piston rod portion and the second        face of the double-face piston.    -   A piston for an internal combustion engine, the piston including        a cylindrical first piston portion having a first diameter, a        cylindrical second piston portion of the first diameter, a        cylindrical third piston portion of a second diameter less than        the first diameter, and located between the first piston portion        and the second piston portion, and wherein the first piston        portion is configured such that prior to assembly, the first        piston portion is separate from the second piston portion, a        continuous, gapless piston ring circumscribing the third piston        portion, where the piston ring is configured such that when        heated the piston ring deforms in an axial direction of the        piston.    -   wherein the third piston portion defines a slot between the        first piston portion and the second piston portion.    -   wherein, prior to assembly, the third piston portion is integral        with the first piston portion, and the second piston portion is        non-integral with the third piston portion.    -   a groove in the outer peripheral wall of the piston the groove        having a first edge and a second edge spaced from the first        edge.    -   a piston ring in the groove, the piston ring having a shape that        meanders within the groove, such that the shape of the piston        ring differs from a shape of the groove and such that the piston        ring does not substantially fill the groove, and wherein the        piston ring is constructed of a material that when subjected to        heat causes a shape of the meanderings to change, thereby        enabling the piston ring to expand in an axial direction of the        piston, between the edges of the groove.    -   wherein the meanderings are in the shape of a wave.    -   wherein peaks of the wave alternatively extend toward opposing        edges of the groove.    -   wherein the piston ring is constructed such that when subjected        to heat, the piston ring tends to expand in an axial direction        of the piston rather than radially.    -   wherein the piston ring has an undulating axial cross section        and a circular radial cross section.    -   wherein the piston ring includes a plurality of staggered, flat        abutment surface portions on axially opposite faces.    -   wherein the flat abutment surface portions are configured to        seat alternately on opposite edges of the groove.    -   wherein a gap between the first and second edges of the groove        allows for axially-directed expansion and contraction of the        piston ring while maintaining a circular radial cross section of        the piston ring having a substantially constant outer diameter.    -   wherein the piston ring is formed with an undulating axial cross        section including a plurality of staggered, flat abutment        surface portions on axially opposite faces thereof, the flat        abutment surface portions being configured to seat alternately        on the first and second edges of the groove with portions of the        piston ring in between the flat abutment surface portions being        spaced from the edges of the groove.    -   a piston formed of an assembly of separate pieces including a        pair of piston end disks each having a first outer diameter, a        piston center disk having a second outer diameter smaller than        the first outer diameter, and wherein the center disk is        configured to cause a thermal gap between the pair of piston end        disks.    -   further including the piston center disk having a hardness that        is different from the piston end disks.    -   wherein the piston center disk is integrally formed with one of        the pair of piston end disks.

Various alterations and modifications may be made to the disclosedexemplary embodiments without departing from the spirit or scope of thedisclosure as embodied in the following claims. For example, the burnedgases produced by the engine 10 may be used for driving a turbo charger.The compressed air introduced into the cylinder may be pressurized by anexternal compressor that is driven by the reciprocating piston rodportions extending from opposite ends of the cylinder. Other variationsmay include imparting a swirl effect to the gases introduced into thecylinder by changing the angle of the inlet ports and of the outletports so that gases are not directed radially into or out of thecylinder.

What is claimed is:
 1. A linear reciprocating engine, comprising: acylinder having a first combustion chamber at a first end thereof and asecond combustion chamber at an opposing second end thereof; a firstcylinder head located at an end of the first combustion chamber; asecond cylinder head located at an end of the second combustion chamber;a double-faced piston slidably mounted within the cylinder andconfigured to move in a first stroke from the first end of the cylinderto the second end of the cylinder, wherein the first stroke includes afirst expansion stroke portion during which chemical energy fromcombustion in the first combustion chamber is converted into mechanicalpower of the piston, and a first momentum stroke portion during whichthe piston continues to move toward the second end of the cylinder andgases are exchanged between the first combustion chamber and a locationoutside the cylinder; and wherein the cylinder and the double-facedpiston are sized such that a total distance the piston is enabled totravel during the first stroke exceeds by at least a quarter width ofthe piston a distance the piston is enabled to travel during the firstexpansion stroke portion of the first stroke.
 2. The engine according toclaim 1, wherein the first cylinder and the second cylinder are sizedsuch that the exceeding distance the piston is able to travel enablesrelease of energy during the momentum stroke portion after the firstexpansion stroke portion ends.
 3. The engine according to claim 1,wherein the cylinder and the double-faced piston are sized such that thefirst expansion stroke portion of the first stroke on a first side ofthe piston as the piston moves from the first end of the cylinder to thesecond end of the cylinder coincides with at least one of a scavengingphase and a gas boost phase on a second side of the piston.
 4. Theengine according to claim 1, wherein the cylinder and the double-facedpiston are sized such that the first momentum stroke portion of thefirst stroke on a first side of the piston as the piston moves from thefirst end of the cylinder to the second end of the cylinder coincideswith a compression phase in the second combustion chamber on a secondside of the piston.
 5. The engine according to claim 1, wherein thedouble-faced piston is configured to move in a second stroke from thesecond end of the cylinder toward the first end of the cylinder, andwherein the cylinder and the double-faced piston are sized such that thesecond stroke includes a second expansion stroke portion during whichchemical energy from combustion in the second combustion chamber isconverted into mechanical power of the piston, and a second momentumstroke portion during which the piston continues to move toward thefirst end of the cylinder and gases are exchanged between the secondcombustion chamber and a location outside the cylinder; and wherein thecylinder and the piston are sized such that a total distance the pistontravels during the second stroke exceeds by at least a quarter length ofthe piston a distance the piston travels during the second expansionstroke portion of the second stroke.
 6. The engine according to claim 5,wherein the second expansion stroke portion of the second stroke on asecond side of the piston as the piston moves from the second end of thecylinder to the first end of the cylinder coincides with at least one ofa scavenging phase and a gas boost phase on a first side of the piston.7. The engine according to claim 5, wherein the second momentum portionof the second stroke on a second side of the piston as the piston movesfrom the second end of the cylinder to the first end of the cylindercoincides with a compression phase in the first combustion chamber on afirst side of the piston.
 8. The engine according to claim 1, furtherincluding: a first piston rod portion extending from a first face of thedouble-faced piston within the first combustion chamber to a firstlocation outside the cylinder; a second piston rod portion extendingfrom an opposing second face of the double-faced piston within thesecond combustion chamber to a second location outside the cylinder; atleast one recess in the first piston rod portion, the at least onerecess forming a passageway configured to communicate gas flow at leastbetween the first combustion chamber and the first location outside thecylinder; at least one recess in the second piston rod portion, the atleast one recess forming a passageway configured to communicate gas flowat least between the second combustion chamber and the second locationoutside the cylinder; and at least one port in a peripheral side wall ofthe cylinder located substantially midway between the first end and theopposing second end of the cylinder, the at least one port beingconfigured to communicate gas flow between the first combustion chamberand outside the cylinder when the piston is on the second combustionchamber side of the at least one port, and being configured tocommunicate gas flow between the second combustion chamber and outsidethe cylinder when the piston is on the first combustion chamber side ofthe at least one port.
 9. The engine according to claim 8, wherein thepassageways in the first and second piston rod portions are configuredto intake gases into the first and second combustion chambers,respectively, and the at least one port in the peripheral side wall ofthe cylinder is configured to serve as a common exhaust for the firstcombustion chamber and the second combustion chamber.
 10. The engineaccording to claim 1, wherein the cylinder and the double-faced pistonare sized such that a total distance the piston is enabled to travelduring the first stroke exceeds by at least half the width of the pistona distance the piston is enabled to travel during the first expansionstroke portion of the first stroke.
 11. The engine according to claim 1,wherein the cylinder and the double-faced piston are sized such that atotal distance the piston is enabled to travel during the first strokeexceeds by at least three quarters the width of the piston a distancethe piston is enabled to travel during the first expansion strokeportion of the first stroke.
 12. The engine according to claim 1,wherein the cylinder and the double-faced piston are sized such that atotal distance the piston is enabled to travel during the first strokeexceeds by at least the width of the piston a distance the piston isenabled to travel during the first expansion stroke portion of the firststroke.
 13. A linear reciprocating engine, comprising: a cylinder havinga first combustion chamber at a first end thereof and a secondcombustion chamber at an opposing second end thereof; a first cylinderhead located at an end of the first combustion chamber; a secondcylinder head located at an end of the second combustion chamber; adouble-faced piston slidably mounted within the cylinder and configuredto move in a first stroke from the first end of the cylinder to thesecond end of the cylinder, and a second stroke from the second end ofthe cylinder to the first end of the cylinder, wherein each of the firststroke and the second stroke includes an expansion stroke portion duringwhich chemical energy from combustion in one of the first combustionchamber and the second combustion chamber is converted into mechanicalpower of the piston, and a momentum stroke portion during which thepiston continues to move toward a respective end of the cylinder andgases are exchanged between one of the first combustion chamber and thesecond combustion chamber and a location outside the cylinder; andwherein the cylinder and the piston are sized such that a total distancethe piston is enabled to travel in each of the first and second strokesexceeds by at least a quarter width of the piston a distance the pistonis enabled to travel during compression of gases on one side of thepiston.
 14. The engine according to claim 13, wherein the cylinder andthe piston are sized such that an expansion stroke portion of the firststroke on a first side of the piston as the piston moves from the firstend of the cylinder to the second end of the cylinder coincides with atleast one of a scavenging phase and a gas boost phase on a second sideof the piston.
 15. The engine according to claim 13, wherein thecylinder and the piston are sized such that a momentum stroke portion ofthe first stroke on a first side of the piston as the piston moves fromthe first end of the cylinder to the second end of the cylindercoincides with compression of gases in the second combustion chamber ona second side of the piston.
 16. The engine according to claim 13,further including: a first piston rod portion extending from a firstface of the double-faced piston to a first location outside thecylinder; a second piston rod portion extending from a second face ofthe double-faced piston to a second location outside the cylinder; atleast one recess in the first piston rod portion, the at least onerecess in the first piston rod portion forming a passageway configuredto communicate gas flow between the first combustion chamber and thefirst location outside the cylinder; at least one recess in the secondpiston rod portion, the at least one recess in the second piston rodportion forming a passageway configured to communicate gas flow betweenthe second combustion chamber and the second location outside thecylinder; and at least one peripheral exhaust port in a side wall of thecylinder, the at least one peripheral exhaust port configured tocommunicate gas flow between the first combustion chamber and outsidethe cylinder when the piston is on the second combustion chamber side ofthe at least one exhaust port, and being configured to communicate gasflow between the second combustion chamber and outside the cylinder whenthe piston is on the first combustion chamber side of the at least oneexhaust port.
 17. The engine according to claim 16, wherein thepassageways in the first and second piston rod portions are configuredto intake gases into the first and second combustion chambers,respectively, and the at least one peripheral exhaust port in the sidewall of the cylinder is configured to exhaust gases from the first andsecond combustion chambers, respectively.
 18. The engine according toclaim 13, wherein the cylinder and the piston are sized such that atotal distance the piston is enabled to travel in each of the first andsecond strokes exceeds by at least half the width of the piston adistance the piston is enabled to travel during compression of gases onone side of the piston.
 19. The engine according to claim 13, whereinthe cylinder and the piston are sized such that a total distance thepiston is enabled to travel in each of the first and second strokesexceeds by at least three quarters the width of the piston a distancethe piston is enabled to travel during compression of gases on one sideof the piston.
 20. The engine according to claim 13, wherein thecylinder and the piston are sized such that a total distance the pistonis enabled to travel in each of the first and second strokes exceeds byat least the width of the piston a distance the piston Is enabled totravel during compression of gases on one side of the piston.